The invention relates to methods of making superconductors having epitaxial layers.
Superconductors are used in a variety of applications. Often, the mechanical integrity of a superconductor can be enhanced by forming a multilayer article that includes a layer of superconductor material and a substrate layer, but the use of a substrate can present certain complications.
Chemical species within the substrate may be able to diffuse into the layer of superconductor material, potentially resulting in a loss of superconductivity. Moreover, the coefficients of thermal expansion as well as the crystallographic spacing and orientation of the substrate and the superconductor layer can be different, causing the article to peel apart during use.
To minimize these complications, a buffer layer can be disposed between the substrate and the superconductor layer. The buffer layer should reduce diffusion of chemical species between the substrate and the superconductor layer, and the buffer layer should have a thermal coefficient of expansion that is about the same as both the substrate and the superconductor layer. In addition, the buffer layer should provide a good crystallographic match between the substrate and the superconductor.
One approach to controlling the crystallographic properties of a layer has been to use epitaxy. An epitaxial layer is a layer of material that is grown on the surface of a substrate such that the crystallographic orientation of the layer of material is determined by the lattice structure of the substrate. By epitaxy is also meant materials with ordered surfaces whether formed by conventional epitaxy or graphoepitaxy. Epitaxial layers have been grown using physical vapor deposition (PVD), chemical vapor deposition (CVD) and sputtering techniques.
Typically, PVD involves the evaporation of a solid material and transfer of the vapor to the substrate surface in a diffuse gas beam in which only a small portion of the total amount of evaporated solid may reach the substrate surface. Thus, the material usage efficiencies obtained with PVD can be low. In addition, PVD is usually performed at a chamber pressure of at most about 1xc3x9710xe2x88x924 torr, so the flux of evaporated solid at the substrate surface can be small, resulting in low epitaxial layer growth rates.
In CVD, one or more reactant gases within the chamber adsorb to the substrate surface and react to form the epitaxial layer with product gases desorbing from the substrate surface. Generally, the reactant gases reach the substrate surface by convection and diffusion, so the *material usage efficiencies can be low. Furthermore, CVD is typically conducted at a chamber pressure of at least about 0.1 torr, and, to grow epitaxial layers at these elevated pressures, relatively high substrate temperatures are usually used. Thus, the selection of substrate materials used in CVD can be limited.
Sputtering methods of growing epitaxial layers can also be limited by the aforementioned considerations.
When using PVD, CVD or a sputtering technique, the quality of the epitaxial layer can depend upon the chemical nature of the substrate surface during layer growth. For example, contaminants present at the substrate surface can interfere with epitaxial layer growth. In addition, native oxides present at the substrate surface can help or hinder epitaxial layer growth. Further, PVD, CVD and sputtering methods can be ineffective at providing control of the chemical nature of the substrate surface during layer growth, so the epitaxial layers formed by these techniques can be of poor quality.
The invention features a low vacuum vapor deposition process for producing a superconductor article with an epitaxial layer. The epitaxial layer is disposed on a substrate that can be of non-identical composition.
In one aspect the invention features a method that includes the steps of placing a textured or crystallographically oriented target surface of a substrate typically including contaminant materials, in a low vacuum environment, and heating the target surface (substrate surface) to a temperature which is greater than the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment under otherwise comparable conditions. A layer-forming stream, including an inert carrier gas and a dispersion of a first species (layer forming gas), which is a chemical component of the desired epitaxial layer, is directed at a positive velocity greater than about 1 m/sec toward the substrate surface through the low vacuum environment. A second species (conditioning gas) is provided in the low vacuum environment, directed toward the substrate surface at a velocity substantially similar to the velocity of the layer-forming stream, and reacted with one or more of the species present in the substrate surface, for example, a contaminant material. This reaction conditions the substrate surface and promotes nucleation of the epitaxial layer. A desired material chemically comprising the first layer forming gas is deposited from the stream onto the substrate surface to form an epitaxial layer. A layer of an oxide superconductor is then deposited on the epitaxial layer.
As used herein, xe2x80x9clayer forming gasxe2x80x9d refers to a gas that can adsorb to a surface and become a component of an epitaxial layer. A layer forming gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like.
As used herein, xe2x80x9cconditioning gasxe2x80x9d refers to a gas that can interact with a surface to remove surface contaminants, remove undesired native oxides present at the substrate surface or form desired native oxides or other components at the substrate surface. A conditioning gas can be formed of atoms, molecules, ions, molecular fragments, free radicals, atomic clusters and the like.
In another aspect, the invention features a method of making a superconductor. The method includes growing, at a chamber pressure of at least about 1xc3x9710xe2x88x923 torr, an epitaxial buffer layer on a substrate having a temperature that is about the same as a PVD epitaxial growth threshold temperature for a chamber pressure of at most about 1xc3x9710xe2x88x924 torr. The method also includes depositing a superconductor material or a precursor of a superconductor material on the epitaxial buffer layer.
A xe2x80x9cprecursor of a superconductor materialxe2x80x9d as used herein refers to a material which can undergo subsequent treatment to become a superconductor. For example, subsequent to heating a particular temperature, a precursor of a superconductor material can become a superconductor material.
For a given epitaxial layer, the xe2x80x9cPVD epitaxial growth threshold temperature for a chamber pressure of at most about 1xc3x9710xe2x88x924 torrxe2x80x9d refers to the minimum substrate temperature that can be used to grow the epitaxial layer at a chamber pressure of at most about 1xc3x9710xe2x88x924 torr, typically at most about 1xc3x9710xe2x88x925 torr, using a PVD with a diffuse gas beam.
As used herein, a xe2x80x9cdiffuse gas beamxe2x80x9d refers to a gas beam in which less than about 50% of any layer forming gas in the gas beam is incident at the substrate surface.
In still another aspect, the invention features a method of making a superconductor. The method includes growing an epitaxial buffer layer on a substrate surface at a rate of at least about 50 Angstroms per second in a vacuum chamber having a pressure of at least about 1xc3x9710xe2x88x923 torr. The method also includes depositing a layer of superconductor material or a precursor of a superconductor material on the epitaxial buffer layer.
In a further aspect, the invention features a method of making a superconductor. The method includes growing an epitaxial buffer layer on a substrate surface by exposing the substrate surface to a gas beam having a layer forming gas, wherein at least about 75% of the layer forming gas in the gas beam is incident at the substrate surface. The method also includes depositing a layer of superconductor material or a precursor of a superconductor material on the epitaxial buffer layer.
In still a further aspect, the invention features a method of making a superconductor. The method includes forming an epitaxial buffer layer by exposing a surface of a substrate to a conditioning gas that interacts with the substrate surface to form a conditioned substrate surface and exposing the conditioned substrate surface to a gas beam having a layer forming gas that becomes a component of an epitaxial buffer layer on the conditioned substrate surface. The exposing steps are performed at a pressure of at least about 1xc3x9710xe2x88x923 torr. The method further includes depositing a layer of a superconductor material or a precursor of a superconductor material on the buffer layer.
A xe2x80x9cconditioned surfacexe2x80x9d herein denotes a surface from which a substantial portion of surface contaminants or undesired native oxides have been removed by a conditioning gas, or on which a desired native oxide or other compounds has been formed by a conditioning gas.
In a still further aspect, the invention relates to a method of making a superconductor. The method includes growing an epitaxial superconductor layer on a surface of a material having a temperature that is about the same as a PVD epitaxial growth threshold temperature for a chamber pressure of about 1xc3x9710xe2x88x924 torr. The growing step is performed in a vacuum chamber having a pressure of at least about 1xc3x9710xe2x88x923 torr, and the material can be a buffer layer or a single crystal.
In one aspect, the invention features an article that includes an epitaxial buffer layer and a superconductor layer disposed on the buffer layer. The buffer layer has a pore density of less than about 500 pores per square millimeter.
As used herein, xe2x80x9cpore densityxe2x80x9d refers to the number of pores at the surface of an epitaxial layer per unit area of the surface of the epitaxial layer.
In another aspect, the invention features an article that includes an epitaxial buffer layer and a superconductor layer disposed on the buffer layer. A surface of the buffer layer has inclusions with an average particle size diameter less than about 1.5 micrometers.
As used herein, xe2x80x9cinclusionsxe2x80x9d refer to surface defects, such as second phase particles, that can act as locations of initiation of non-epitaxial growth at a surface.
The substrate material may be ceramaceous, such as an oxide, metallic, such as a metal or alloy, or intermetallic. It chemically comprises one or more substrate species, and at least one substrate species is different from the layer forming gas. In addition to the substrate species, the species present in the substrate surface may include native, oxides of one or more of the substrate species, and adsorbed surface contaminants.
Since the epitaxial layer typically includes a metal, the layer forming gas is usually a metal vapor. If the epitaxial layer includes more than one species, such as a metal oxide, the layer forming gas can comprise more than one gas. For example, if the epitaxial layer is formed of cerium oxide, the cerium vapor and oxygen can be used in the gas beam.
The conditioning of a substrate surface can occur by exposing the substrate surface to a background pressure of a conditioning gas. Alternatively, the conditioning of a substrate surface can occur by exposing the surface to a gas beam that includes a conditioning gas. Such gas beams can be directed gas beams or diffuse gas beams, and the conditioning gas included therein may have a high velocity or a low velocity. In some embodiments, conditioning of the substrate can occur by exposing the substrate surface to a background pressure of a conditioning gas and a conditioning gas that is included in the gas beam.
When using a background pressure of a conditioning gas to condition a substrate surface, the vacuum chamber typically has a partial pressure of the conditioning gas of at least about 0.5 percent, preferably from about 0.5 percent to about 10 percent, and more preferably from about 2 percent to about 6 percent. Thus, for example, if the vacuum pressure were about 100 millitorr, the partial pressure of hydrogen in the chamber can be at least about 0.5 millitorr.
In embodiments in which a conditioning gas is included in a gas beam, the conditioning gas may be introduced in the same stream as the layer forming gas or in a separate stream. In some embodiments, other species may also be introduced which are reactive either with the layer forming gas or with one or more additional species present in the substrate surface, but the stream containing the layer forming gas preferably contains only one other reactive species. Additional reactive species are preferably introduced in separate streams of substantially similar velocity. Large dissimilarities in stream velocities can favor one set of species reactions at the expense of the others, and are generally to be avoided. The desired epitaxial layer material may be the layer forming gas or the reaction product of the layer forming gas with one or more of the other species introduced into the low vacuum environment. The desired material may be ceramaceous, metallic, or intermetallic. Moreover, the desired material can be an oxide superconductor.
The substrate surface can be exposed to the conditioning gas and the layer forming gas in series or in parallel. For parallel processes, one portion of the substrate surface is conditioned while the epitaxial layer is grown on another portion of the substrate surface.
The chemical nature of the conditioning gas depends upon species present at the surface on which the epitaxial layer is grown. For example, if the surface has an undesired native oxide present at its surface, the conditioning gas can be a reducing gas, such as hydrogen. Alternatively, the conditioning gas can be an oxidizing gas, such as oxygen, if it is desirable to form a native oxide at the surface or if sulfur or carbon are contaminants present at the surface.
The velocities of the layer forming gas, conditioning gas and additional gaseous species dispersed in their streams should be positive., that is directed toward the substrate surface. In some embodiments, the velocities are greater than about 1 m/sec. Velocities as high as supersonic may be used. Velocities in the range of from about 10 m/sec to about 400 m/sec are preferred at typical low vacuum conditions. In a preferred embodiment, the velocity of the layer forming gas is originally high and is reduced from a high velocity to a low velocity prior to contact with the substrate surface, which in essence reduces its kinetic energy. The reduction in velocity is accomplished by interference from the low vacuum environment and also from a boundary layer of carrier gas which forms at the substrate surface. The decrease in the kinetic energy assists in thermal equilibration of the desired material with the substrate. While the high initial velocity of the gaseous species aids in the efficient transport of the layer forming gas to the substrate surface and thereby facilitates higher deposition rates, low velocity gases may also be effectively used in the scope of this invention. The term high velocity refers to positive velocities which approach at least about sonic levels, i.e the speed of sound at operating vacuum levels, such as 100-400 m/sec. The term low velocity refers to positive velocities of less than 10 m/sec and more generally less than 100 m/sec, but greater than 1 m/sec.
The term elevated temperature(s) refers to temperatures which are sufficient to allow diffusion of the desired material to low energy equilibrium sites after arrival at the substrate surface. The range of elevated temperatures will vary depending upon the materials involved and other specifics of the deposition process, such as vacuum level and deposition rate. Although this is a low vacuum process, the lower temperature limit is set as the threshold temperature for forming an epitaxial layer of the desired material on the substrate material in a high vacuum environment at the predetermined deposition rate. As used herein, the term low vacuum (or high pressure) refers to a vacuum pressure which is achievable using mechanical pumping systems or a pressure greater than or equal to about 1xc3x9710xe2x88x923 torr, and the term high vacuum (or low pressure) refers to pressure less than about 1xc3x9710xe2x88x925 torr of base vacuum. Temperatures found suitable for forming epitaxial layers in the invention correspond to temperatures that are suitable for forming epitaxial layers with pressures that are from about 103 to 106 lower, and preferably from about 103 to 104 lower, than pressures used with methods that involve diffuse gas beams. Thus, for example if a pressure of about 1xc3x9710xe2x88x921 torr is used in the invention to form an epitaxial layer of a desired material on a given substrate material at a particular growth rate, the substrate temperature can be the same as would be used at a pressure of from 1xc3x9710xe2x88x921 to 1xc3x9710xe2x88x927 torr to grow the same epitaxial layer under the same conditions with a diffuse gas beam. An upper end of the elevated temperature range is set as 90% of the melting point of the selected substrate material. Typically the substrate surface is heated to a temperature which is less than the prior art threshold temperature for forming an epitaxial layer of the desired material on the substrate material in the low vacuum environment at the predetermined deposition rate.
In some embodiments, an additional conditioning gas which can be reactive with the layer forming gas is provided.
In other embodiments, the conditioning gas is reactive with the layer forming gas as well as one or more species present at the substrate surface. In embodiments in which the substrate material is metallic and the epitaxial layer is an oxide of the layer forming gas, the conditioning gas may be chosen to be more oxidizing with respect to the layer forming gas than with respect to a native oxide of the substrate material. In embodiments in which the substrate and epitaxial layer are both metallic, the conditioning gas may be reducing with respect to both. In some embodiments, a layer forming gas can also serve as a conditioning gas. For example, oxygen can be used to remove impurities and/or an oxide of the substrate surface as well as form an oxide epitaxial layer.
When a substrate of preferred crystallographic orientation is used, such as a cubic textured substrate, deposition of an epitaxial layer onto the substrate according to the process of this invention provides a composite material which is suitable for preparing a superconductor composite. The cubic texture of the substrate is transferred to the epitaxial layer which, in turn may transfer this orientation to a superconducting oxide layer. The superconducting oxide layer may be deposited onto the substrate and epitaxial layer using the process of this invention.
The invention allows the application of high vacuum-like evaporation processes to the fabrication of large scale substrate areas without limitations which are typically imposed by a high vacuum system. Material usage efficiencies are typically increased from less than 50 percent to over 95 percent by the directed layer-forming deposition streams. Furthermore, the directed layer-forming stream in the low vacuum environment reduces the uncertainty associated with the formation of epitaxial films on metallic substrates. The deposition stream can produce a surface boundary layer which enhances the nucleation and growth of epitaxial films on the substrate surface and reduces the temperatures required for epitaxial film growth. The addition of at least one conditioning gas, as allowed by the low vacuum operation, can be used to disrupt a native surface oxide on substrate materials which allows the template of the crystallographically oriented substrate, preferably a cubic textured alloy, to be directly available for the depositing atoms of the preselected material. The layer forming gas and the conditioning gas can be selected to thermodynamically favor specific compounds and materials to improve epitaxial film growth. A consistent process is developed as well, which can eliminate the system to system variation in the deposition of epitaxial films in high vacuum systems. The high energy, low vacuum approach of the present invention also provides the opportunity to use a single manufacturing method for both the buffer layer and the superconducting film layer of a composite article without high vacuum processing.
For epitaxial layers, it is generally disadvantageous to have a high pore density or inclusions with a large average particle size. The invention can provide epitaxial layers having low pore densities and/or inclusions with small particle sizes. The epitaxial layers preferably have a pore density of less than about 500 pores per square millimeter, more preferably less than about 250 pores per square millimeter, and most preferably less than about 130 pores per square millimeter. The epitaxial layers preferably have inclusions with an average particle size of less than about 1.5 micrometers, more preferably less than about 1 micrometer, and most preferably less than about 0.5 micrometers.
In embodiments in which a conditioning gas is included in a gas beam, the gas beam which impinges upon the substrate surface should include a sufficient amount of conditioning gas to condition the substrate surface and allow the epitaxial layer to grow. Preferably, the gas beam that impinges upon the substrate surface includes from about 0.5 molar percent to about 10 molar percent conditioning gas, more preferably from about 1 molar percent to about 8 molar percent conditioning gas, and most preferably form about 2 molar percent to about 6 molar percent conditioning gas.
In some embodiments, it can be advantageous to form the epitaxial layer at a fast rate. In these embodiments, the epitaxial layer is preferably grown at a rate of at least about 50 Angstroms per second, more preferably at least about 100 Angstroms per second, and most preferably at least about 150 Angstroms per second. These growth rates can be achieved at a vacuum chamber pressure of at least about 1xc3x9710xe2x88x923 torr.
The epitaxial layers can be formed without using electron beams or ion beams.